Add Flexbox class to isolate browser-specific CSS3 support.
Add compatibility methods from ControlRenderer.

Allows com.qwirx.ui.Renderer to be used as a Renderer for a goog.ui.Control.
Add problematic code for BorderLayout without CSS3.

Doesn't properly detect components resizing due to delayed loading of
CSS stylesheets or content changes.
Use CSS3 flexbox to manage the layout if possible.